1. Field of the Invention
The invention pertains to the field of fluid check valves. More particularly, the invention pertains to cushioned check valves.
2. Description of Related Art
Water hammer, also known as hydraulic shock, can occur in piping systems that carry a high momentum fluid when rapid changes in momentum of the fluid take place, for example, when a valve in the system is abruptly closed and the fluid stops flowing. As flowing fluids generally have a constant density and mass, changes in fluid momentum result from, for example, changes in fluid flow velocity, a cessation of fluid flow, or a reversal of flow direction causing retrograde flow. When a valve in the system is closed and fluid flow within the system suddenly stops, the change in fluid flow velocity causes a shockwave to form and propagate through the fluid and piping structures that carry the fluid. The shockwave may be characterized, physically and mathematically, as a transient high pressure pulse moving through the fluid flow system.
When the shockwave impacts valve gates and other solid structures, the energy carried by the high pressure of the shockwave is transferred to these solid structures. The shockwave pressure impacting piping and valve structures is undesirable, as it is a source of unwanted acoustic noise, vibration, and extreme pressure gradients that may cause significant mechanical stress on pipes, valves, and other fixtures. In extreme cases, pipes may burst from excessive pressure extremes associated with a shockwave, or conversely may implode at a location as a result of shockwave formation at another location.
In some systems, for example lift stations bringing a fluid such as water or sewage from one elevation to a higher elevation, check valves are often used to prevent or retard retrograde fluid flow when pumping systems are turned off or valves are closed. Changes in pump status when a pump is turned off, and closing of the check valve in these systems, may cause significant hydraulic shock, particularly when large diameter pipes and large differences in elevation are involved.
In the prior art, various solutions to mitigate hydraulic shock have been employed. In some solutions, the fundamental mitigation approach has been to either provide an alternative energy absorbing pathway for fluids to flow in, so that shockwave energy is dissipated when hydraulic shock occurs. In other approaches, the rate at which changes in flow velocity occur is regulated in order to prevent the formation of shockwaves at their source, or minimize the energy and extreme pressure increases associated with shockwaves.
Water towers, or vertical water column shunts, commonly provide alternative energy absorbing pathways. Fluids being pumped from a lower elevation to a higher elevation tend to reverse direction and produce retrograde flow back to the lower elevation when pumps are turned off or valves are closed. An open topped water tower or vertical water column located between the two elevations and at a higher elevation than a check valve, allows the retrograde flow and shockwave energy to be redirected upwardly into the tower or water column, against the force of gravity, thus harmlessly absorbing the shockwave energy and preventing hydraulic shock.
Buffers, such as tanks filled with a compressible gas, may also be incorporated in fluid systems to absorb shockwave energy and pressure, and reduce or eliminate hydraulic shock. Retrograde flow redirected toward the tank increases the fluid pressure in the tank, which in turn compresses the compressible gas, and shockwave energy is thus absorbed and then fed back into the fluid system by the initial compression and subsequent expansion of the gas after the fluid system returns to nominal operating pressures.
In other mitigation approaches, basic considerations such as valve closing rates, pump rate of stop, and length of straight-line piping between elevations may be adjusted to also reduce or eliminate hydraulic shock.
In the case of pump stoppages, hydraulic shock occurs when a pump stops suddenly, causing a sudden change in fluid flow velocity in piping connected to the pump. Adding a massive flywheel to the pump, for example, slows the rate at which pumping stops when power to the pump is removed, and thus slows the rate of change of fluid flow velocity, so that shock waves are not produced, or their pressure amplitude is minimized
Alternatively, short continuous straight-line runs of piping between elevations, such as serpentine pathways, may also minimize hydraulic shock. Bends in a pipeline decrease the total mass of fluid flowing together in a section of pipe in a given direction, and therefore also decrease the total momentum of the fluid flowing in that section of pipe.
Since basic system design considerations may not always adjust to mitigate hydraulic shock, or are cost prohibitive, cushioned check valves have been developed that change the rate of check valve closing to mitigate hydraulic shock. In these prior art cushioned check valves, fluid being pumped from a lower elevation to a higher elevation may stop flowing toward the higher elevation, and reverse direction toward the lower elevation as valves are closed, or pumps stop pumping, while a check valve closes.
For example, a check valve in-line in a lift station between a lower elevation and a higher elevation requires a certain amount of time to close when movement of fluid toward the higher elevation stops, and retrograde flow begins to carry a valve disk backward toward a valve seat until the check valve closes and stops the retrograde flow. The fluid being pumped may therefore develop significant retrograde flow velocity toward the lower elevation that causes hydraulic shock with a significant amount of energy and pressure when the valve disk ultimately closes, and the retrograde flow abruptly stops.
Shockwave mitigation in prior art systems using check valves has focused on forcing the check valve to close at a faster rate than would otherwise occur based on retrograde fluid flow alone forcing a valve disk backward against a valve seat. Ideally, if the valve disk can be made to close at the moment flow stops, and before retrograde flow through the check valve may begin, no hydraulic shock would occur. In actual practice, this ideal timing of the check valve closing is not always achievable.
However, if the rate at which a check valve closes is increased, the faster valve disk closing rate shortens the time retrograde fluid flow has to accelerate toward the closing check valve, and decreases retrograde flow velocity at the moment of check valve closure. Thus, rapid valve closing rates may significantly reduce shockwave energy and pressure, and mitigate hydraulic shock.
In one prior art construction, the Surgebuster® check valve (Mfg.: ValMatic® Valve & Mfg. Corp.) implements a leaf spring on a valve disk that biases the valve disk toward a valve seat, and actively closes the check valve when fluid flow rate through the check valve is reduced below a certain value. Additionally, the check valve is constructed with a short stroke between a fully open position and a fully closed position, shortening the distance the valve disk must move when closing, and further accelerating the check valve closure rate. As a result, fluids flowing through the check valve do not have time to develop significant retrograde velocity toward the valve disk when the valve closes, and hydraulic shock is mitigated.
In some prior art check valve constructions, counter-weights are used to accelerate the rate at which a valve disk closes, reducing retrograde flow velocity and shockwave energy and pressure. Accelerating the rate at which a check valve closes in this manner requires additional moving parts that increase manufacturing costs, and create failure points that may result in leaks over time or require shorter service intervals.
In some prior art check valve constructions, acceleration of the valve disk closure rate may also involve components, such as actuators or complex valve seat orientations, that interfere with the fluid flow path through the check valve, and thus reduce over-all flow rates through check valves of a given diameter.
In addition, these mitigation approaches may increase the force necessary to maintain the valve disk in an open position, and thus increase the hydraulic impedance of the fluid flow system. This increased impedance is undesirable as it requires other components, such as pumps downstream from the check valve, to work at higher pressures.
In some other prior art fluid flow systems, such as engine intake manifolds, undesirable transient high pressure pulses may occur in fluid flow channels carrying gaseous phase fluids. Transient high pressure pulses in some engine intake manifolds are undesirable, as an increase in pressure of a gaseous fuel-air fluid mixture in the manifold may result in pre-ignition of the fuel-air mixture, and damage to the manifold or other engine components, including catastrophic failure.
One approach to mitigating transient high pressure pulses in gaseous phase fluid flow systems has been previously described by Kennedy (U.S. Pat. No. 5,186,198, “Intake Manifold Relief Valve”, issued 1993). In this prior art construction, transient high pressure pulses forming shockwaves are mitigated in a diesel engine intake manifold via a reactive relief valve. Specifically, a piston under a spring bias moves according to pressure changes in the manifold that exceed a certain rate and amplitude, allowing rapid excessive pressure changes in the manifold to be safely vented to the ambient environment. The relief valve is also designed to rapidly close after venting so that a nominal operating pressure of the intake manifold may be consistently maintained.